Factors in Pregnancy Blocking: Age and Reproductive Background of Females: Numbers of Strange Males

Abstract

Pregnancy blocking experiments involving 731 female laboratory mice (control and experimental) indicate: (1) an increased resistance among older females; (2) an equal susceptibility among parous and non-parous females; and (3) a lower pregnancy rate among females exposed to six males than those exposed to one or three males.

The Bruce effect revisited: is pregnancy termination in female rodents an adaptation to ensure breeding success after male turnover in low densities?

“…2 ). Overall, these results confirm earlier findings: in many rodent species, females breeding for the first time were more likely to interrupt their pregnancy than parous females after male turnover (Stehn and Jannett 1981 ; Clulow et al 1982 , but see Chipman and Fox 1966 ). Moreover, younger females are more likely to interrupt a pregnancy than older females (Clulow and Langford 1971 ; Heske 1987 ).…”

“…The combination YY-n, which occurs at intermediate densities, may be intermediate also in the rate of delayed births. There are indications that females of different cohorts are differentially likely to show the Bruce effect: first time breeders more likely interrupted a pregnancy than experienced breeders (Stehn and Jannett 1981 ; Clulow et al 1982 , but see Chipman and Fox 1966 ); and younger females more likely than older females (Clulow and Langford 1971 ; Heske 1987 ).…”

Pregnancy blocking in the vole, Microtus agrestis

“…4E) and caging the female in cages soiled by males or females (Exps 5C and 5D), all failed to produce any significant reduction in pregnancy rate. Pregnancy is readily blocked in newly mated female mice on exposure to strange males in situations similar to those that were unsuccessful in the present study (Bruce, 1959(Bruce, , 1960aParkes & Bruce, 1962;Chipman & Fox, 1966b;Dominic, 1966). Pheromones in the urine of the male are believed to be the effective stimuli causing pregnancy block in the mouse, and pregnancy and pseudopregnancy are blocked by the application of urine from the strange male to the external nares of newly mated females (Dominic, 1964(Dominic, ,1966.…”

Who Bears the Costs of Interspecific Competition in an Age-Structured Population?

“…The two age-and season-dependent life history patterns may be reflected in other life history traits, such as the onset of breeding (Fairbairn 1977), the susceptibility to pregnancy inter- Manuscript received 17 April 2002;revised 6 February 2003;accepted 19 March 2003. Corresponding Editor: J. S. Brown. ruption (Chipman andFox 1966, Stehn andJannet 1981), or over-winter survivorship (Zejda 1964, Tkadlec andZejda 1998b). However, investigating these fitness consequences of life history patterns in wild populations is difficult, as it is nearly impossible to age live voles once they have started breeding (Boonstra 1994).…”